I. Field of the Invention: This invention relates generally to medical instrumentation, and more particularly to an electronic device for measuring and indicating the percentage of one or more constituents of arterial blood.
II. Discussion of the Prior Art: Various systems are disclosed in the prior art for measuring the percentage of various constituents in arterial blood. For example, in the Wilber U.S. Pat. No. 4,407,290 assigned to Biox Technology, Inc., of Boulder, Colo., there is described an oximeter which can be used, non-invasively, for measuring the ratio of oxygenated hemogloblin in arterial blood by providing first and second light sources of differing wavelength which shine light through a body member with the transmitted or reflected light being picked up by a photo detector. The signal picked up by the detector is effectively modulated by the pulsatile flow of blood through the area being sensed, and then the pulse train is divided into separate channels in which further signal processing operations are performed. The Biox device thus requires a number of channels of substantially similar electronic devices equal to the number of light sources required to uniquely identify the constituents being measured. Generally speaking, the number of radiation sources of different wavelength needed to measure n constituents is n+1. This replication of electronic circuitry in plural channels naturally increases the cost of the instrument and also can adversely affect the device's accuracy, given the fact that component values in one channel can shift with time relative to corresponding components in another.
The Nielsen U.S. Pat. No. 4,167,331 assigned to the HewlettPackard Company of Palo Alto, Calif., likewise describes a pulse oximeter in which plural light sources are sequentially turned on an off and transmitted through a body part in which arterial blood flows. The transmitted or reflected (scattered) light is picked up by a photo-sensitive device and the resulting pulse modulated signal is fed through a logarithmic amplifier to produce a voltage which is a logarithmic function of the current from the sensor. This voltage is then divided so as to be processed by separate signal processing hardware in separate individual channels, the number of channels corresponding to the number of blood components (and individual light sources) employed in the system. Thus, like the Biox device, the Hewlet-Packard device also tends to be costly as the number of channels increases.
The Biox and Hewlett-Packard devices also are subject to error introduced by extraneous light sources or other noise which is picked up by the photo detecting device and fed through the plural channels along with the desired signal produced by the separate light sources utilized in the apparatus. Such extraneous light sources have been known to impact the accuracy of the instrument by as much as ten percent or even more.